Plant and method for vacuum distillation of hydrocarbon liquids

ABSTRACT

A vacuum distillation system and method utilizing an auxiliary, low capacity vacuum producing ejector operated in parallel with a primary ejector during the winter months enables significant reduction in the absolute pressure of a vacuum distillation column. Operation of a vacuum distillation tower at lower absolute pressures results in increased yield of desirable vacuum distillation products.

CROSS-REFERENCED AND RELATED APPLICATIONS

This application is a divisional application of U.S. patent applicationSer. No. 10/401,617 filed in the United States Patent and TrademarkOffice on Mar. 26, 2003. Said U.S. patent application Ser. No.10/401,617 is, in its entirety, incorporated into this application byreference.

FIELD OF THE INVENTION

The present invention relates to the field of hydrocarbon refining bythe method of vacuum distillation.

BACKGROUND OF THE INVENTION

Vacuum distillation of petroleum hydrocarbons is a well known refiningprocess commonly utilized in the art to minimize thermal cracking ofheavier fractions of crude oil and obtain lighter desired products.Di-stilling these heavier materials under vacuum, that is lowerpressure, decreases the boiling temperature of the various hydrocarbonfractions in the feed and therefore minimizes thermal cracking of thesefractions. In conventional vacuum distillation systems, distillation iscarried out in a vacuum column under pressures typically in the range of25 to 100 millimeters of mercury (mmHg). It is important in such systemsto reduce pressure as much as possible to improve vaporization.Vaporization is enhanced by various methods such as the addition ofsteam at the furnace inlet and at the bottom of the vacuum distillationcolumn. Vacuum is created and maintained using cooling water condensersand steam driven ejectors. The size and number of ejectors andcondensers used is determined by the vacuum needed and the quantity andquality of vapors handled. Three ejector stages are usually required fora distillation column flash zone pressure of 25 mmHg. In a conventionalsystem of this type, the first stage condenses the steam and compressesnon-compressible gases, while the second and third stages remove thenon-condensable gases from the condensers. The vacuum produced islimited to the vapor pressure of the water used in the condensers. Ifcolder water is supplied to condensers, a lower absolute pressure can beobtained in the vacuum tower.

In contemporary conventional vacuum distillation systems, the mostcommon design includes a condenser upstream of the ejectors, known as apre-condenser, to reduce the size and steam consumption of the vacuumsystem FIG. 1 illustrates a pre-condenser type vacuum system. In thistype of system, the lowest achievable pressure is the partial pressureof the process vapor in the pre-condenser, which is determined by thetemperature of the condensing fluid, usually water. In a pre-condensertype system, the process vapor from the pre-condenser is usually 80 to90 percent water so the lowest pressure achievable is about 15 percentgreater than the vapor pressure of water at the equilibrium vapor outlettemperature, which as noted is equal to the temperature of thecondensing liquid, usually water. In warm, humid climates, such as theUS Gulf Coast where the average wet bulb temperature in the summertimeis 80° F., the lowest achievable pressure in a pre-condenser type vacuumsystem is about 50 mmHg when condenser-cooling water is available onlyat ambient temperatures. Adding more ejectors or increasing the capacityof the existing ejectors in the system does not result in lowerdistillation column pressure because of the high percentage of water inthe process vapor.

To further minimize thermal cracking, it is desirable to achievepressures in vacuum distillation columns lower than the 40 to 60 mmHgtypical of pre-condenser type vacuum systems. This problem is addressedin vacuum distillation system designs known as “deep-cut technology”,which involves installation of primary ejectors upstream of anycondensers in the process flow scheme. FIG. 2 illustrates a “deep-cut”type vacuum distillation system in this type of system, the vacuumdistillation column overhead vapor is compressed by the primary ejectorwhich allows the column to operate at low absolute pressures in therange of 5 to 20 mmHg. As illustrated in FIG. 2, the large primaryejector discharges first into a large condenser, known as a firstinter-condenser. The first inter-condenser must condense not only thecolumn stripping and furnace lift steam, but also the motive steam usedto power the primary ejector.

In deep-cut type vacuum distillation systems, the primary ejectors aredesigned for a specific suction load and discharge pressure, also knownas backpressure. The process or suction load to the primary ejector isdetermined by the vacuum column process design requirements (principallyprocess steam injection rate and desired vacuum gas oil make) while thebackpressure is determined primarily by the design of the firstinter-condenser. In the art, the normal practice is to design theejectors to operate under worst case conditions, that is, when thecooling water available is the warmest and the heat exchanger in thecondensing system is fouled. Thus, when cooling water temperature iscolder than such “worse case” design conditions, such as in the coolseason months in sub-tropical regions, extra condensing capacity isavailable and the first inter-condenser. This extra condensing capacityresults in a lower than design backpressure in the primary ejector.

In sub-tropical regions of the Northern hemisphere, such as the US GulfCoast, the average temperature of available cooling water during theyear varies between about 60° F. and 85° F. During the cool seasonmonths, lower cooling water temperatures result in a pressure in thefirst inter-condenser can be expected to range between about 35 mmHg and60 mmHg, whereas during the warm season months, the expected absolutepressures in the first inter-condenser typically are about 20 percent to40 percent higher. Pressure in the first inter-condenser representsbackpressure on the primary ejector, which, as noted, is designed tooperate efficiently under the worst-case conditions prevalent during thewarm season months. Thus, the primary ejector is not typically designedto take advantage of lower backpressures available as a result of lowertemperature cooling waters available during the cooler season months.

FIG. 4 depicts a characteristic performance curve for a primary ejectorand illustrates and provides an example of the problem to be solved bythe present invention. Thus, as illustrated in FIG. 4, as the process orsuction load to the primary ejector increases, so does the suctionpressure and therefore the column pressure. In the example shown in FIG.4, the primary ejector is designed for a process load in the range of8,500 to 10,500 pounds per hour to achieve a suction pressure in therange of a 13 to a 17 mmHg. This standard performance curve does notshow the effect of backpressure from the first inter-condenser on theejector. Under this circumstances lowering the backpressure will nothave any measurable effect on the ejectors suction pressure. Thus, theextra condensing capacity available from lower temperature cooling wateravailable during the cool season months does not result in a lowercolumn pressure. Available methods to reduce the column pressure, suchas decreasing the suction load by decreasing the column stripping steaminjection rate or reducing the furnace lift steam injection rate, hasthe effect of decreasing efficiency of the vacuum column and reduces therecovery of desirable gas oil product. Another method to reduce thevacuum column pressure would be to install larger primary ejectors;however, larger primary ejectors requires installation of facilities forinjection of more steam, installation of larger condensers and systemsfor circulating larger amounts of cooling water. Installation of suchsystems would, of course, involve undesirable capital expenditure.Accordingly, the object of the present invention is to take advantage ofseasonal variations in cooling water temperatures, by installing anadditional ejector that can be seasonally operated in parallel with theprimary ejector that has the effect of reducing the process or suctionload to the primary ejector and therefore reduces the vacuum columnpressure. See FIG. 4.

SUMMARY OF THE INVENTION

The present invention is directed to a plant and method for vacuumdistillation of a liquid comprising a vacuum distillation column havinga pipeline for receiving a heated feed, a gaseous vapor dischargepipeline and at least one liquid discharge pipeline for discharging atleast one liquid fraction; a first condenser; a primary vacuum producingejector having an inlet and connected to the gaseous vapor dischargepipeline and an outlet end connected to the first condenser; anauxiliary vacuum producing ejector having an inlet end connected to thegaseous vapor discharge pipeline and an outlet end connected to thefirst condenser; at least one second stage vacuum producing ejectorhaving an inlet end that can be connected to various process componentssuch as the first condenser, a separate second condenser, a condensatecollection vessel or a discharge pipeline that routes distillationproduct to other refining systems. In a preferred embodiment two or moresecond stage ejectors and condensers are connected in series to formcondensate product that is discharged to a condensate collection vessel,usually termed a seal drum. The auxiliary ejector, herein denoted awinter ejector, as a capacity of about 2.0 percent to 20 percent of thecapacity of the primary ejector, preferably a capacity of about 5percent to 15 percent of the primary ejector capacity and morepreferably about 10 percent of the primary ejector capacity. More thanone auxiliary ejector can be installed to increase operatingflexibility; for example, if a 10 percent load reduction is desired two5 percent auxiliary ejectors may be installed in parallel with a primaryejector. The winter ejector(s) is/are operated in parallel with theprimary ejector during the cool season months when the averagetemperature of available cooling water, for example, in the US Gulfcoast region is about 70° F. or about 15 degrees lower than averagecooling water temperatures available during the warm season months. Inother regions of the World, ambient, temperatures and cooling watertemperature will vary significantly from this US Gulf Coast regionexample. The operative factors are the relative difference between coolseason and warm season cooling water temperatures and the design limitsof the primary ejector. When the winter ejector is operated, the load tothe primary ejector will decrease resulting in decreased pressure to thevacuum distillation column. Generally, a decrease in load to the primaryejector will result in an equivalent decrease in vacuum column pressure.For example, if a 10 percent decrease in load to the primary ejector isachieved, the corresponding column pressure will also be reduced 10percent. Such cool season reduction in vacuum column pressure willincrease the recovery of heavy gas oil from heavy petroleum hydrocarbonfeeds.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts a conventional pre-condenser type of vacuum distillationsystem. FIG. 2 depicts a conventional “deep-out” type of vacuumdistillation system utilizing a primary ejector upstream of thecondensing systems. FIG. 3 depicts a typical vapor pressure regime inthe inter-condenser in a deep-cut type vacuum distillation system overan annual cycle. FIG. 4 depicts the characteristic performance curve fora primary ejector in a deep-cut type vacuum distillation system. FIG. 5depicts a preferred embodiment of the vacuum distillation system of thepresent invention showing the installation of the winter ejectorconnected in parallel with the primary ejector.

DETAILED DESCRIPTION OF THE INVENTION

Refer to FIG. 5. A system for vacuum distillation of a hydrocarbonliquid, comprises a vacuum distillation column 10, a pipeline forreceiving a heated feed 20, a gaseous vapor discharge pipeline 30, atleast one liquid discharge pipeline 40, a first condenser 45, a primaryvacuum producing ejector 50 having an inlet end 50A and an outlet end50B, connected to the first condenser 46 via pipeline 60, and auxiliaryvacuum producing ejector 70 having an inlet end 70A connected to thegaseous vapor discharge pipeline 30 and outlet end connected to thefirst condenser 45, either via pipeline 50C in one embodiment ordirectly in an alternative embodiment. The system further comprises atleast one-second stage vacuum producing ejector 800 having an inlet endconnected to the first condenser 45 via pipeline 90 and an outlet endconnected to at least one-second condenser 100 via pipeline 110.Preferably, the system includes at least two-second stage vacuumproducing ejectors 80, 81 and at least two second stage condensers 100,101. Condensers 45, 100 and 101 are connected to a condensate collectionvessel 120 via pipelines 120A, 1208, 120C, 120D and 120E. The desiredgas oil product is recovered from vessel 120 via line 130 and vacuumresiduum is recovered from the vessel via line 140.

The primary ejector 50 is designed to receive a process vapor or suctionload via gaseous vapor discharge pipeline 30. The auxiliary ejector 70has a suction load capacity varying between about 2.0 percent to 20percent of the capacity of the primary ejector, preferably the auxiliaryejector capacity is between about 5 percent to 15 percent of thecapacity of the primary ejector and most preferably the auxiliaryejector capacity is about ten percent of the capacity of the primaryejector. The auxiliary ejector 70 is turned on during the cool seasonmonths by means of valves 71 and 72 and motive steam is received viavalve 51 and lines 51A and 51B. In the warm season, the primary ejector50 is operated independently of the auxiliary ejector 70 by closingvalves 71, 72 and 51 and has a warm season outlet pressure typicallyabout 20% to 40% greater than the cool season outlet pressure. In thecool season, the primary ejector 50 is operated in parallel with theauxiliary ejector 70T which has the effect of reducing the inletpressure of the primary ejector 50 by about 2 percent to about 20percent. Preferably, the primary ejector 50 inlet pressure is reduced byabout 5 to 15 percent and, most preferably, by about 10 percent when itis operated in parallel with the auxiliary ejector 70 during the coolseason months.

A process for vacuum distillation of a hydrocarbon liquid during thecool season utilizing the above described vacuum distillation systemcomprises the steps of feeding a heated hydrocarbon liquid to the vacuumdistillation column 10, maintaining a low absolute pressure in thevacuum distillation column by means of at least one primary ejector 50and at least one auxiliary ejector 70 operating in parallel with theprimary ejector 501 evacuating a first hydrocarbon vapor product fromthe vacuum distillation column 10; circulating a cooling water streamthrough a first condenser 45 and at least one second condenser 100; atleast partially condensing the first hydrocarbon vapor product in thefirst condenser 45 to form a first condensate and a second hydrocarbonvapor product; discharging the first condensate to a condensatecollection vessel 120 and evacuating the second hydrocarbon vaporproduct by at least one second stage ejector 80, 81 to at least onesecond condenser 100, 101, product to form a second condensate anddischarging the second condensate to a condensate collection vessel 120,a second condenser and discharging a product from the condensatecollection vessel 120 via a product discharge pipeline 130 or 140 forrouting distillate products to other refining processes. In the methodof this invention, the auxiliary ejector has a suction load capacitybetween about 2 percent to 20 percent of the suction load capacity ofthe primary ejector 50, preferably, the suction load capacity of theauxiliary ejector 70 is about 5 to 15 percent of the suction loadcapacity of the primary ejector 50 and most preferably the suction loadcapacity of the auxiliary ejector 70 is about 10 percent of the suctionload capacity of the primary ejector 50. The method further comprisescirculating to the first condenser 45 a cooling water stream having atemperature at least 10° F. below the first condenser 45 design maximumcooling water temperature, which is typically based upon warm seasonmaximum cooling temperatures. Generally, the greater the temperaturedifferential between the design maximum cooling water temperature andthe cool season cooling water temperature, the greater the vaporpressure reduction in the first condenser 45, which facilitatesoperation of the auxiliary ejector and a reduction in suction load tothe primary ejector. For example, in the U.S. Gulf Coast Region, themethod comprises a cool season cooling water stream having a temperaturebetween 60° F. and 85° F. and a warm season cooling water streamtemperature between about 80° F. and 90° F. Thus, in this sub-tropicalregion, there can be as much as a 30° F. temperature differentialbetween cool season and warm season cooling water temperatures. Theauxiliary, winter ejector is operated in parallel with the primaryejector when the cool season water temperature is a least 10° F. belowthe first condenser 45 maximum design point. In this example, as shownin FIG. 3, during the cool season, vapor pressure in the first condenser45 varies between about 35 mmHg and 50 mmHg. The method furthercomprises operating the auxiliary ejector during the cool season monthswhen the first condenser 45 pressure is between about 35 mmHg and 50mmHg, which has the effect of reducing the suction load of the primaryejector to about 2 percent to 20 percent below its design point.Preferably, the suction pressure of the primary ejector is reduced tobetween 10 mmHg and 15 mmHg, and most preferably the suction pressure ofthe primary ejector is reduced to between about 11 mmHg and 13 mmHg.

Thus, it can be seen from the above description that the object of thepresent invention is to provide and improved system and method forvacuum distillation of a liquid, in particular, heavy hydrocarbonliquids. The system and method involves installation of an auxiliaryvacuum producing ejector that can be seasonally operated in parallelwith the primary vacuum producing ejector to reduce absolute pressuresin the vacuum distillation column by as much as 10 to 15 percent duringthe cool season months. The system and method takes advantage of thecooler condenser cooling waters that are available during the coolseason months.

The invention can be used and the petroleum refining, petrochemical andother industries where vacuum processing of liquid products is required.It is possible to economically integrate the invention process intoconventional vacuum distillation systems. It should be noted thatvarious changes and amendments could be made in the details within thescope of the claims set forth below without departing from the spirit ofthe claimed invention. It should therefore be understood that theclaimed invention should not be limited to the specific details shownand described.

1. A method for vacuum distillation of a hydrocarbon liquid during coolseason months comprising the steps of: a. feeding the hydrocarbon liquidto a vacuum distillation column; b. evacuating a first hydrocarbon-vaporproduct from the distillation column by a primary vacuum producingejector and at least one auxiliary vacuum producing ejector, wherein theprimary vacuum producing ejector and the at least one auxiliary vacuumproducing ejector are connected in parallel to a source of motive stem;c. circulating a cooling water stream through a first condenser and atleast one second condenser; d. at least partially condensing the firsthydrocarbon-vapor product in the first condenser to form a firstcondensate and second hydrocarbon vapor product; e. discharging thefirst condensate to a condensate collection vessel; f. evacuating thesecond hydrocarbon vapor product by at least one second stage vacuumproducing ejector to the at least one second condenser to form a secondcondensate; g. discharging the second condensate to the condensatecollection vessel.
 2. The method of claim 1 wherein the auxiliary vacuumproducing ejector of step b is connected to a source of motive steam inparallel with the primary vacuum producing ejector, a first valve isconnected between the vacuum distillation column and the at least oneauxiliary vacuum producing ejector, a second valve is connected betweenthe at least one auxiliary ejector and the first condenser and a thirdvalve is connected between the at least one auxiliary ejector and thesource of motive steam.
 3. The method of claim 1 or 2 wherein theauxiliary vacuum producing ejector has a suction load capacity about 2percent to 20 percent of the suction load capacity of the primary vacuumproducing ejector.
 4. The method of claim 1 or 2 wherein the auxiliaryvacuum producing ejector has a suction load capacity about 5 percent to15 percent of the suction load capacity of the primary vacuum producingejector.
 5. The method of claim 1 or 2 wherein the auxiliary vacuumproducing ejector has a suction load capacity about 10 percent of thesuction load capacity of the primary vacuum producing ejector.
 6. Themethod of claim 1 or 2 wherein the auxiliary vacuum producing ejector isoperated when the cooling water stream temperature during the coolseason is at least 10° F. lower than the cooling water streamtemperature during the warm season.
 7. The method of claim 1 or 2wherein the first condenser pressure during the cool season is betweenabout 55 percent and 90 percent of the first condenser pressure duringthe warm season.
 8. The method of claim 1 or 2 wherein the firstcondenser pressure average during the cool season is about 65 to 75percent of the first condenser pressure average during the warm season.9. The method of claim 1 or 2 wherein the auxiliary vacuum producingejector is operated in parallel with the primary ejector.